The following abbreviations are frequently used in the draft:
PSNT: Public Switched Telephone Network
TCP: Transport Control Portocol
UDP: User Datagram Portocol
RTP: Real Time Portocol
IP: Internet Portocol
VoIP: Voice Over IP
CD: compressor/de-compressor
CDMA: Code Division Multiplexing Access
MAC: Medium Access Control
RLC: Radio Link Control
GSM: Global System for Mobile communications
GPRS: General Packet Radio Service
1. Field of the Invention
This invention relates to an apparatus and method for telecommunications using Internet Protocol, and relates especially to an advantageous data compression arrangement.
In a wireless telecommunications network operating a packet switching technology, it would advantageous to be able to offer voice and data services using the Internet, but the need to transmit the combined headers of Real-time Transport Protocol (RTP), User Datagram Protocol (UDP) and Internet Protocol (IP) in each packet header is a disadvantage. The three headers are respectively 20, 8 and 12 bytes per packet, and this 40 byte load is nearly double the voice payload of 23 bytes for 20 milliseconds in the voice coding system known as GSM FR. It is well known that wireless resource/bandwidth is more expensive than landline arrangements, and the overload of the large header load is a serious drawback.
2. Brief Description of the Prior Art
One solution as disclosed by S Casner and V Jacobson. xe2x80x9cCompressing IP/UDP/RTP Headers for Low-Speed Serial Linksxe2x80x9d, RFC2508, ftp://ftp.isi.edu/innotes/rfc2508.txt is to compress the RTP/UDP/IP headers. A protocol is defined to compress and decompress the headers, and a reduction to between 2 and 5 bytes can be achieved. However, a major disadvantage is that if a compressed packet is lost it must be re-transmitted and the provision of an error-recovery facility is essential, and therefore the round-trip-time must be short if quality is to be maintained. Further, for many radio networks in which the radio bearer capacity is designed for circuit voice service so the link layer PDU size is designed to match voice payload, it is difficult to put even 2-5 more bytes into one Medium Access Control (MAC) block, so that one voice frame with its compressed header may have to be transmitted in two MAC blocks, requiring extra radio resource or there may be loss of voice quality; yet again, handover may change the compression/decompression point in the network, requiring the compression state to be re-established, which needs a CONTEXT_STATE message then a FULL_HEADER packet to be sent, which causes delay and packet loss.
In S Petrack, Ed Ellesson, Framedwork for Compressed RTP, Preliminary IETF draft, presented on rem-con mailing list. Feb. 1996, http://www.mbonecom/lists/rem-conf.1996Q1/0259.html, it is suggested that the two time related fields (RTP sequence number and timestamp) can be compressed using a 1-byte xe2x80x9ctimeclickxe2x80x9d number and a separate RTP session control is suggested to signal the static parts of the headers out of band, but no details are given of how to achieve this.
According to the invention, a mobile station for a packet switching radio network which includes a Voice over Internet Protocol (VoIP) application layer and Internet Protocol Stacks including Real Time Protocol, Transport Control Protocol, User Datagram Protocol and Internet Protocol layers, further comprising a compressor/decompressor to compress or remove RTP/UDP/IP headers of voice packets, and means to send voice data and call signalling data separately and in different data formats to the link layer and the air interface of the mobile station, in which voice packets for VolP applications are sent as a link layer payload directly without going through IP layers.
In one arrangement the compressor/decompressor is arranged between link layer (which is above the physical layer in the air interface) and the IP layer, and receives both the voice data and the call signalling data.
In an alternative arrangement the compressor/decompressor is arranged between the air interface and the VoIP applications layer, and receives the voice data.
Also according to the invention a packet switching radio network comprising a plurality of mobile stations as defined above and at least one network element in which there is compressor/decompressor means arranged to receive signals relating to compressed voice data.
Further the present invention provides a method of operating a packet switching radio network to provide voice services comprising separating the voice data from the call signalling and other data, whereby each voice packet containing voice data is provided with compressed or removed RTP/UDP/IP headers, in which the voice packets for VoIP applications are sent as a link layer payload directly without going through IP layers.
Yet further according to the invention, a first method of compressing and decompressing headers for a packet switching network comprises providing in each compressed header a cyclically-reset timeclick_number representing the sampling time of the packet payload; increasing the timeclick_number by 1 for each sample duration time, counting the reset cycles, and from the count of reset cycles and a received timeclick_number, providing a sequence number and timestamp for providing a decompressed header.
Yet further according to the invention, a second method of compressing and decompressing headers for a packet switching network comprises removing combined RTP/UDP/IP headers and placing data in RLC/MAC payload; and decompressing received packets by use of an internal clock to obtain a timestamp value, and increasing the sequence number by 1 for consecutive packets.